Erythropoietin (Epo) is a 30-kDa, heavily glycosylated protein produced primarily by the kidney. It is the principal factor that regulates erythropoiesis. The production of Epo by kidney cells is increased in response to hypoxia or anemia. The cDNAs for Epo have been cloned from many species. The mature proteins from the various species are highly conserved, exhibiting greater than 80% sequence identity at the amino acid level.
The biological effects of Epo are mediated by the Erythropoietin receptor (Epo R). At the protein sequence level, the human Epo R is approximately 83% identical to the mouse protein. As a result of alternative splicing of the Epo R gene, cDNA clones encoding truncated soluble forms of Epo R have been found. Recombinant soluble Epo R binds Epo with high affinity and is a potent Epo antagonist.